It seems the field is of data type Text (with content which represents a date), since it's crawled as Text. If you want to use the field for dates with time, you have to set the data type of the field (the data type of the column in the list or library) to Date and Time. Then it will be crawled as a real Date and Time data type and the mapping will be possible.

When you change the column data type from Text to Date and Time a new crawled property will be created with data type Date and Time.

Perhaps you should try surfacing the lastModified coulmn in your page directly?
Maybe a small section in the footer that says: Page Published with a DateTimeField surfacing the column.
This way the crawler will correctly identify the underlying datatype of the column, create the mapped property with the right variant type and you can map it as you desire.

Put a feild in the page that references the coulmn. Like this one: <SharePointWebControls:FieldValue FieldName="[FieldGuid]" runat="server"></SharePointWebControls:FieldValue >
– GavinBMay 25 '11 at 20:37

Did you try to create a new crawled property with the appropriate VariantType (Date) by PowerShell command New-SPEnterpriseSearchMetadataCrawledProperty. Also, maybe the DateTime value has to be in the correct format to be parsable.